Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
2001-05-17
2004-03-30
Gulakowski, Randy (Department: 1746)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S047000
Reexamination Certificate
active
06713207
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a membrane electrode assembly applied to a solid polymer fuel cell. More particularly, a membrane electrode assembly applied to a solid polymer fuel cell which is suitable for use in a transportable compact power supply, a vehicle-mounted power source, a cogeneration system and the like, and a solid polymer fuel cell using the assembly.
2. Description of Related Art
A solid polymer fuel cell is a fuel cell in which a solid polymer electrolyte membrane is used as an electrolyte. The basic unit thereof, a unitary cell, is constituted of a pair of electrodes bonded respectively to both surfaces of a solid polymer electrolyte membrane (hereinafter, referred to as an “membrane electrode assembly”.) Each of the electrodes has a two-layer structure consisting of a diffusion layer and a catalyst layer, and the latter is provided on a surface in contact with the solid polymer electrolyte membrane.
The diffusion layer is a layer for supplying a reactant gas to the catalyst layer and exchanging electrons, and it is made from materials having porosity and electron conductivity. The catalyst layer is a layer for causing a catalyst contained therein to initiate electrode reaction. Yielding the electrode reaction requires a three-phase interface where three phases of an electrolyte, a catalyst, and a reactant gas coexist. Therefore, the catalyst layer is typically constituted of a catalyst or a catalyst supported by a catalyst carrier and a porous layer including an electrolyte having the same composition as the solid polymer electrolyte membrane.
By the way, non-crosslinked perfluoro-based electrolytes, typified by Nafion (a registered trademark for products manufactured by E.I. du Pont de Nemours and Company), and a variety of hydrocarbon-based electrolytes have been known as solid polymer electrolytes for use in a solid polymer fuel cell. Any of these, however, needs water for appearance of ion conductivity. Therefore, if the operation of the fuel cell enters a dry condition, so-called dry-up occurs, meaning that water content of a solid polymer electrolyte membrane decreases, and so does electrical conductivity of the membranes, which may become a cause of lowered output of the fuel cell.
On the other hand, if the operation of the fuel cell enters a wet condition, excessive water builds up inside electrodes. In addition, when protons are conducted within a solid polymer electrolyte membrane from one electrode (anode) to the other electrode (cathode), the water also transfers along with the protons to the side of the cathode (this will hereinafter be referred to as “water electroosmosis”.) Besides, in the cathode, water is produced via electrode reaction. If the water is left standing, so-called flooding occurs, meaning that the three-phase interface in the catalyst layer is clogged with the water, which may become a cause of lowered output of the fuel cell.
Accordingly, it is necessary to maintain a solid polymer electrolyte membrane in an appropriate wet state so as to ensure high output with stability from a solid polymer fuel cell. Conventional types of a solid polymer fuel cell have employed a method by which reactant gases supplied to electrodes are humidified with an aid of auxiliary machinery such as a steam generator or a mist atomizer while the amount of humidification is controlled so as to adjust water content of the solid polymer electrolyte membrane (this will hereinafter be referred to as “water control”.) Besides, there has also been a known method whereby water is injected directly into a reactant gas passages formed within a separator.
However, to make a solid polymer fuel cell more compact and lightweight, it is desired to improve water control property of a membrane electrode assembly so as to lower a degree to which the water control depends on the auxiliary machinery. In order to achieve this, it is considered effective to convert a solid polymer electrolyte membrane into a thin film of high strength. This is because enhancing the strength of a solid polymer electrolyte membrane permits the membrane to be a thin film, which facilitates maintaining the whole of the membrane in a uniform wet state.
As a method of converting a solid polymer electrolyte membrane into a thin film, there have been known several methods including a method whereby an electrolyte is made to contain another crosslinkable polymer for reinforcement (e.g. see Japanese Patent Unexamined Publication Nos. 06(1994)-76838 and 10(1998)-340732), a method whereby a coating of fluorine-based monomer is applied to a reinforcing material constituted of porous fibers and then the monomer is polymerized to introduce ion-exchange groups thereto (e.g. see Japanese Patent Examined Publication No.04(1992)-58822), and a method whereby an electrolyte membrane is bonded to a parfluorocarbon polymer woven fabric by thermal compression to be converted into a multilayer film (e.g. see Japanese Patent Unexamined Publication No. 06(1994)-231780.)
In addition, there has been a known method whereby ion clusters in a perfluoro sulfonic acid membrane are inter-knitted through an inorganic glasslike network in silicon oxide phase (siloxane polymer), silicon oxide+titanium oxide phase, zirconium oxide phase, or the like so as to make a hybrid membrane by sol-gel reactions initiated by immersion of the perfluoro sulfonic acid membrane in an alcohol solution containing alkoxide such as tetraethoxysilane, a mixture of tetrabutyltitanate and tetraethoxysilane, tetrabutylzirconate or the like (e.g. see Journal of Applied Polymer Science, vol.55, p.181 (1995).) Besides, there has also been a known method whereby finely granulated silica and/or fibrous silica fiber is added in order to increase water content and ion conductivity of a solid polymer electrolyte membrane and those of catalyst layers (e.g. see Japanese Patent Unexamined Publication No. 06(1994)-111827.)
Using any of the above-described methods to make a solid polymer electrolyte membrane thinner in thickness and higher in strength, the whole of the membrane can easily be maintained in a comparatively uniform wet state. Flooding or dry-up in a fuel cell, however, arises in dependence on not only the water control property of a solid polymer electrolyte membrane but also that of an electrode.
Accordingly, any of those conventional methods can suppress the flooding and the dry-up resulting from a solid polymer electrolyte membrane to a certain extent, but it is difficult to suppress flooding and dry-up resulting from an electrode. In addition, as an example of giving attention to improving the water control property of an electrode in order to relieve a load on auxiliary machinery at the time of water control, Japanese Patent Unexamined Publication No. 06(1994)-111827 mentions a technique to improve electrical conductivity by adding granulated or fibrous silica. And yet, this technique is not effective enough to greatly improve electrical conductivity since it does not allow of introduction of silica into the inside of conductive paths which are still finer, meaning that this technique does not contribute to the water control property of an electrode.
Such a membrane electrode assembly is commonly produced by application of pressure bonding by hot-pressing to the surfaces of diffusion layers coated with a paste containing a catalyst or a catalyst supported by a catalyst carrier and an electrolyte in the form of a solution. However, conventional types of membrane electrode assemblies present such a problem that bonding failure may occur or that the conductive paths are apt to be discontinuous, because those assemblies are made by merely pressing and bonding electrolytes in the form of solutions contained in catalyst layers to electrolytes in the form of membranes.
On the other hand, in order to solve the problem that output of a fuel cell is lowered by flooding which is caused by excessive water building up particularly inside a cathode with the operating condition of the fuel cell b
Kawasumi Masaya
Morimoto Yu
Tsusaka Kyoko
Gulakowski Randy
Kabushiki Kaisha Toyota Chuo Kenkyusho
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wills Monique
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